Dry gas seals are increasingly popular as shaft seals in turbomachines, for example in turbocompressors. Dry gas seals in the sense of the present invention are shaft seals which have at least two sealing elements extending around the shaft in the circumferential direction (with respect to the axis of rotation) and comprising in each case at least one sealing surface which faces the opposite sealing element, by means of which the dry gas seal seals the intermediate space between a static part, for example a housing, and the rotating part, in particular a shaft. One of the sealing elements rotates with the rotating part and the other sealing element is stationary with respect to the housing. At least one of the sealing surfaces which face each other has projections and/or recesses which act together with a sealing gas to establish a gas film between the two sealing elements on the sealing surfaces, such that the dry gas seal operates, during the intended rotation, without contact between the rotating sealing element and the static sealing element. The dry gas seal requires a clean sealing gas having substantially no liquid components, in order to seal in a damage-free and/or contactless fashion. The recesses and/or projections can have various shapes and are preferably provided on only one sealing surface, preferably on the sealing surface of the rotating sealing element. Recesses can be configured with a U shape, a firtree shape, a T shape or in the shape of a spiral, such that a stable sealing film of the sealing gas is formed between the two sealing surfaces. Decisive advantages result from the use of dry gas seals, for example in compressors, such that the leakage is comparatively small and no lubricating oil is necessary, such as for example in the case of seals working with liquids or in comparison with a labyrinth seal. A sealing gas is regularly applied to a high-pressure side of dry gas seals, wherein a small leakage of this sealing gas passes through the dry gas seal from the high-pressure side to a side of lower pressure—a low-pressure side—where this leakage is generally evacuated into what is termed a vent line. In order also to reduce the consumption of the specially prepared sealing gas, the dry gas seal is generally surrounded by other shaft seals; in particular a labyrinth seal, which on the high-pressure side reduces the outflow of the sealing gas to the side which faces away from the dry gas seal, is generally located on the high-pressure side. The main reason for the serial additional seal, in particular a labyrinth seal, is the sealing effect in the event of the dry gas seal failing. Even those dry gas seals which have a sealing gas supply line as a central supply line of the sealing surfaces are generally protected from contamination by means of a serial additional seal, in particular a labyrinth seal. A process gas, which in the case of a turbocompressor is at an operating pressure, is usually present on the high-pressure side of the dry gas seal. The sealing gas must be at an overpressure with respect to this process gas in order to keep the process gas—which may be contaminated—away from the delicate dry gas seal.
An arrangement having a dry gas seal and a labyrinth seal is already known from, in each case, DE 10 2009 012 038 A1, DE 42 25 642 C1 and EP 0 781 948 A1, but the assembly complexity of these is higher due to the modular separation of the labyrinth seal from the dry gas seal. The fabrication and assembly of these two modules requires increased precision because modern machinery efficiency requirements require tight radial play of the labyrinth seal and precise arrangement of the dry gas seal.